X‐ray photoelectron spectroscopy ‐An introductionAn XPS spectrum consists of peaks corresponding...
Transcript of X‐ray photoelectron spectroscopy ‐An introductionAn XPS spectrum consists of peaks corresponding...
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X‐ray photoelectron spectroscopy ‐ An introduction
Spyros Diplas
SINTEF Industry, Materials Physics -Oslo&
Centre of Materials Science and Nanotechnology, Department of Chemistry, UiO
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Material Characterisation Methods
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What is surface?• What happens at surfaces is extremely important in a vast range of applications from environmental corrosion to
medical implants.
• A surface is really the interface between different phases (solid, liquid or gas).
• We can think of the surface as the top layer of atoms but in reality the state of this layer is very much influenced by the 2 – 10 atomic layers below it (~0.5 – 3 nm).
• Surface modification treatments are often in the range of 10 – 100 nm thick. >100 nm can be thought of as the bulk.
• Surface analysis encompasses techniques which probe the properties in all these ranges.
God made solids, but surfaces were the work of the devil ------Wolfgang Pauli
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• Properties and reactivity of the surface will depend on:• bonding geometry of molecules to the surface • physical topography• chemical composition• chemical structure• atomic structure• electronic state
No one technique can provide all these pieces of information. However, to solve a specific problem it is seldom necessary to use every technique available.
photons
ions
electrons
EMISSION
TRANSMISSION
Interaction with material
EXCITATION
Surface Analysis ‐ Techniques Available
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hν
Photoelectron
2p1/2, 2p3/2
2s
1s
Ekin = hν – EB -
L23
L1
K
EKL2,3L2,3(Z) = EK(Z) – [EL2,3(Z) + EL2,3(Z + 1)]
Internal transition
(irradiative)
Auger electron
XPS‐Basic Principle
valence bandFermi
Vacuum
De-excitationExcitationAn XPS spectrum consists of peaks corresponding to emission of both photoelectrons and Auger electrons
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Auger electron vs x‐ray emission yield
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B Ne P Ca Mn Zn Br Zr
10 15 20 25 30 35 40 Atomic Number
Elemental Symbol
0
0.2
0.4
0.6
0.8
1.0Pr
obab
ility
Auger Electron Emission
X‐ray Photon Emission
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Schematic of an XPS spectrometer
Number of emitted electrons measured as function of their kinetic energy
Al
X-ray source
Electrostatic electron lens Electron
detector
Electron energy analyser
Samplee-
Photon
Slit
Hemispherical electrodes
Slit
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Examples of XPS spectrometers
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Instrument: Kratos Axis UltraDLD at MiNaLab
Analyser
Monochromator
Sample
Detector
X-ray source
X-ray source
e-
e-
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Why is XPS surface sensitive?XPS Depth of Analysis
The probability that a photoelectron will escape from the sample without losing energy is regulated by the Beer‐Lambert law:
Where λe is the photoelectron inelastic mean free path
Attenuation length (λ) ≈0.9 IMFPIMFP: The average distance an electron with a given energy travels between
successive inelastic collisions
Typical electron energies in the XPS spectrum
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Features of the XPS spectrum Primary structure
- Core level photoelectron peaks (atom excitation)- Valence band spectra - CCC, CCV, CVV Auger peaks (atom de-excitation)
Secondary structure
- X-ray satellites and ghosts- Shake up and shake off satellites- Plasmon loss features- Background (slope)
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XPS spectrum ITO
x 104
10
20
30
40
50
60
70
80CPS
1200 1000 800 600 400 200 0Binding Energy (eV)
In 3dSn 3d
O 1s
In 3pSn 3p
In 3sIn 3s
In MNNSn MNN
O KLL
Auger peaks
Photoelectron peaks
In/Sn 4pIn/Sn 4s
C 1s
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Peak width (ΔE)ΔE = (ΔEn
2 + ΔEp2 + ΔEa
2)1/2
Gaussian broadening:
-Instrumental:There is no perfectly resolving spectrometer nor a perfectly monochromatic X-ray source.
-Sample:For semiconductor surfaces in particular, variations in the defect density across the surface will lead to variations in the band bendingand, thus, the work function will vary from point to point. This variation in surface potential produces a broadening of the XPS peaks.
-Excitation process such as the shake-up/shake-off processes or vibrational broadening.
Lorentzian broadening:
The core-hole that the incident photon creates has a particular lifetime (τ) which is dependent on how quickly the hole is filled by an electron from another shell. From Heisenberg’s uncertainty principle, the finite lifetime will produce a broadening of the peak.
Γ=h/τ
Intrinsic width of the same energy level should increase with increasing atomic number
Natural width X-ray source contributionAnalyser contribution
Chemical shift
ΔE(i) = kΔq + ΔVM – ΔR
Initial state contribution
• Δq: changes in valence charge
• ΔVM : Coulomb interaction between the photoelectron (i) and the surrounding charged atoms.
.
final state contribution
• ΔR: relaxation energy change arising from the response of the atomic environment (local electronic structure) to the screening of the core hole
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Chemical shift - Growth of ITO on p c-Si
104 102 100 98
BHF 15 sec + 500°C
0.5 nm
1.5 nm
Si2p
456 454 452 450 448 446 444 442 440
In3d
0.5 nm
1.5 nm
3.0 nm
500 495 490 485 480
Sn3d
0.5 nm
1.5 nm
3.0 nm
Inte
nsity
arbi
trary
uni
ts
Binding Energy (eV)
SiOx
Si In oxide
In
Sn oxide
Sn
3/2 3/2 5/2 5/2
1.5 nm
0.5 nm
BHF 15 sec + 500oC 0.5 nm
1.5 nm
3.0 nm
0.5 nm
1.5 nm
3.0 nm
Si 2p In 3d Sn 3d
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Chemical shift
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Quantification Unlike AES, SIMS, EDX, WDX there are little in the way of matrix effects to worry about in XPS. We
can use either theoretical or empirical cross sections, corrected for transmission function of the analyser. In principle the following equation can be used:
I = J ρ σ K λ
I is the electron intensity J is the photon flux, ρ is the concentration of the atom or ion in the solid, σ s is the cross-section for photoelectron production (which depends on the element and energy being
considered), K is a term which covers instrumental factors, λ is the electron attenuation length.
In practice atomic sensitivity factors (F) are often used: [A] atomic % = {(IA/FA)/Σ(I/F)} Various compilations are available.
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Spin-Orbit Coupling/Splitting
Spin-orbit coupling/ splitting: final state effect for orbitals with orbital angular momentum l> 0. A magnetic interaction between an electron’s spin and its orbital angular momentum.
Example Ti. Upon photoemission an electron from the p orbital is removed - remaining electron can adopt one of two configurations: a spin-up (s=+1/2) or spin-down (s=-1/2) state. If no spin-orbit interaction these two states would have equal energy (degenerated states).
spin-orbit coupling lifts the degeneracy To realise that we need to consider the quantum number, j, the total angular momentum quantum
number. j=l + s where s is the spin quantum number (±½). For a p orbital j=1/2 or 3/2. Thus the final state of
the system may be either p1/2 or p3/2 and this gives rise to a splitting of the core-level into a doublet as shown in the figure above.
Spin-orbit coupling is described for light elements by the Russell-Saunders (LS) coupling approximation and by the j-j coupling approximation for heavier elements
Arb
itrar
y U
nits
468 466 464 462 460 458 456Binding Energy (eV)
Tioxide 2p 2p3/2
2p1/2
The intensity of the peaks is given by the degeneracy
gJ = 2j+1
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Depth profile with ion sputtering
Use of an ion gun to erode the sample surface and re-analyse Enables layered structures to be investigated Investigations of interfaces Depth resolution improved by:
Low beam energiesSmall ion beam sizesSample rotation
SnO2
Sn
Depth500 496 492 488 484 480
Elemental distribution and oxygen deficiency of magnetron sputtered ITO filmsA. Thøgersen, M.Rein, E. Monakhov, J. Mayandi, S. Diplas
JOURNAL OF APPLIED PHYSICS 109, 113532 (2011)
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ITO surface
ITO/Si interface
InIn‐oxide
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XPS-Check list Depth of analysis ~ 5nm All elements except H and He Readily quantified (limit ca. 0.1 at%) All materials (vacuum compatible) Chemical/electronic state information
-Identification of chemical states-Reflection of electronic changes to the atomic potential
Compositional depth profiling by -ARXPS (ultra thin film <10 nm), -change of the excitation energy -choose of different spectral areas-sputtering
Ultra thin film thickness measurement Analysis area mm2 to 10 micrometres